Soil sampling and biogeochemical measurements
Soil samples were collected in early May of 2010. To dismiss within-plot heterogeneity, we collected a relatively large soil core (> 2 Kg) from each plot. A total of 12 soil cores at 15-25 cm depth from warmed plots and their paired control plots were collected using electric drills and were transported to the laboratory on ice. In the lab, we divided each sample into two parts using a sterile saw: one for soil DNA extraction (stored at -80 °C); the other for soil biogeochemical measurements (stored at 4 °C), including bulk density, labile carbon pool (LCP, mainly polysaccharides and cellulose), recalcitrant carbon pool (RCP, mainly lignin), total organic carbon (TOC), soil nitrogen, NH4+-N, and NO3--N. Soil bulk density was calculated as its dry mass (oven-drying at 60 °C for three days) divided by its frozen volume. A two-step hydrolysis procedure separated LCP and RCP. LCP was quantified by a Shimadzu TOC-V CPH PC-Controlled TOC analyzer (Shimadzu Corporation, Tokyo, Japan). RCP was quantified by a PerkinElmer Optima 2000DV ICP-OES spectrometer (PerkinElmer Inc., Waltham, MA, USA) (Rovira & Vallejo, 2002). NH4+-N and NO3--N were measured using anion (AG 1-X8, 20-50 mesh) and cation (AG 50W-X8, 20-50 mesh) exchange bags (Biorad, Hercules, CA, USA). In each sampling plot, a cation bag and an anion bag were placed at a depth of 10 cm during May-September 2010. Then samples in the bags were collected, and inorganic nitrogen components were extracted in the 0.1M HCl/2.0M NaCl solution for an hour in the lab and analyzed on an Astoria Analyzer (Astoria-Pacific Int., Clackamas, OR, USA).